mmc: update sdio_claim_irq documentation

Update documentation for sdio_claim_irq to downgrade the wording
about doing recursive claims in an IRQ handler from 'must not' to
'should not'. This clarifies that recursive claims are supported,
but not the recommended (best) practice

Signed-off-by: Joel Cunningham <joel.cunningham@garmin.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>